1. Field of the Invention
The present invention relates to interface circuits and, particularly, to a display data channel (DDC) interface circuit on a motherboard.
2. Description of the Related Art
DDC is a standard communications channel between a computer and a monitor. A monitor contains a read-only memory (ROM) chip programmed by the manufacturer with information about the graphics modes that the monitor can display. The data in the monitor's ROM is held in a standard format called extended display identification data (EDID). The EDID is a data structure provided by the monitor to describe its capabilities for a graphics card of the computer. With this information, the computer knows what kind of monitor it is connected to. The EDID is defined by the video electronics standards association (VESA). The EDID includes manufacturer name, product type, phosphor or filter type, timings supported by the monitor, monitor size, luminance data and (for digital displays only) pixel mapping data. The EDID information is communicated to the computer over the DDC. The EDID and the DDC enable the computer and the monitor to communicate so that the computer can be configured to support specific features available in the monitor. However, the computer often cannot obtain the EDID because the computer cannot recognize an automatic color killer (ACK) signal output from the monitor.
What is needed, therefore, is a DDC interface circuit which can overcome the above problem.